1. Technical Field
The technical field relates to an imaging apparatus which can change a level of aperture correction dynamically.
2. Related Art
As a process for enhancing a change (edge) in a signal value of an image signal obtained from an image sensor such as a CCD, an aperture correction process is known. For example, JP08-009199A discloses an imaging signal processing apparatus for suppressing an aperture correction for a low brightness signal. When a constant aperture correction is made regardless of a brightness signal, the aperture correction is excessively applied to the low brightness signal so that an uncomfortable image would be generated. The imaging signal processing apparatus disclosed in JP08-009199A suppresses the aperture correction for the low brightness signal to achieve an aperture correction that does not generate an uncomfortable image.
In recent years, lenses for imaging apparatuses have been reduced in size and thinned, imaging devices have been large in size, and their pixels are increased in number. Further, size reduction and low cost of the imaging apparatus accelerate reduction in the number of the lenses and usage of plastic lenses. This decreases a flexibility of a lens design and influences lateral color, distortion, astigmatism, coma and the like, particularly in a peripheral area of an image. As a result, an MTF (Modulation Transfer Function) characteristic in a peripheral area, particularly resolution and contrast at a high spatial frequency tend to decrease. A problem that might be caused on taking a group photo by means of such lenses is described below.
FIGS. 13A to 13C are diagrams for describing a decrease in contrast in the peripheral area of an image. FIG. 13A schematically illustrates a state in which an image of a subject is obtained through an optical system. A frame in the image shows that a face of a person present on a center area of the image is focused on. FIG. 13B is a graph illustrating a relationship between a spatial frequency and a contrast. A horizontal axis represents the spatial frequency, and a vertical axis represents the contrast. A solid line indicates the contrast on the center area of the image, and a broken line indicates the contrast on the peripheral area of the image. The contrast on the peripheral area of the image is lower than that on the center area of the image at a low spatial frequency range, and is noticeably reduced at a high spatial frequency range. Since the resolution is closely related to the contrast at the high spatial frequency range, similarly the resolution is noticeably reduced at the high spatial frequency range. As a result, for people who are present on the peripheral area of the image which is taken as a group photo, both the contrast and resolution particularly at the high spatial frequency are reduced. FIG. 13C is a schematic diagram illustrating a position relationship between a focus lens and an imaging device. When the center area of the image comes into focus, a best focus plane is present on a position contacting an imaging plane at the center of the imaging device, but a gap G1 with respect to the best focus plane is generated at peripheral (edge) area of the imaging device. This is a significant factor for decrease in contrast and resolution on the peripheral area of the image.
FIGS. 14A to 14C are other diagrams for describing the decrease in the contrast on the peripheral area of the image. FIG. 14A schematically illustrates a state in which an image of a subject is obtained through the optical system similarly to FIG. 13A. A frame of the image shows that people present on the peripheral area of the image come into focus. FIG. 14B is a graph illustrating a relationship between a spatial frequency and a contrast similarly to FIG. 13B. The contrast on the center area of the image is lower than that on the peripheral area of the image at the lower spatial frequency range. Therefore, the resolution on the center area of the image is also lower than the resolution on the peripheral area of the image at the lower spatial frequency range. As a result, for people who present on the center area in image of a group photo, both the contrast and the resolution are reduced particularly at the lower spatial frequency range. FIG. 14C is a schematic diagram illustrating a position relationship between the focus lens and the imaging device similarly to FIG. 13C. When the peripheral area of the image comes into focus, a best focus plane is in contact with the imaging plane at the periphery of the imaging device, but a gap G2 with respect to the best focus plane is generated at the center of the imaging device. This is a significant factor for decreases in contrast and resolution on the center area of the image.
Even when the center area of the image comes into focus, an important subject might be present on the peripheral area of the image. In taking of a group photo, generally, both people present on the center area of the image and people present on the peripheral area of the image are equally important. On the other hand, even when the peripheral area of the image comes into focus, an important subject might be present on the center area of the image. In portrait photography and macro photography, it is often the case that a subject present on the center area of the image is important. However in even taking of other type of photograph such as a group photo, it might often occur that important subjects are present on both the center area and the peripheral area of the image. In such cases, both the subject present on the center area of the image and the subject present on the peripheral area of the image are desired to be photographed with high contrast and resolution. Conventional techniques (for example, the technique disclosed in JP08-009199) cannot solve such a problem.